A quantum well designed to detect infrared (IR) light is called a quantum well infrared photodetector (QWIP). QWIPs operate by photo-excitation of electrons between a ground state and an excited state of its quantum wells. In more detail, a quantum well absorbs IR photons. This absorption of IR photons photo-excite electrons from the ground state to the excited state of each quantum well. The excited states of the quantum wells making up a QWIP effectively form an energy transport band (sometimes referred to as the continuum or a miniband). A voltage externally applied to the QWIP operates to sweep out the photo-excited electrons, thereby producing a photocurrent in the continuum.
Quantum wells are grown in a crystal structure. In general, layers of two different, high-bandgap semiconductor materials are alternately grown. The bandgap discontinuity of the two semiconducting materials creates quantized sub-bands in the wells associated with conduction bands. Only photons having energies corresponding to the energy separation between the ground and excited states are absorbed. This is why a QWIP has such a sharply defined absorption spectrum. Note that each well can be shaped to detect a particular wavelength, and so that it holds the ground state near the well bottom, and the excited state near the well top.
A group of QWIPs can be used to form a focal plane array (FPA) in detection applications, where each QWIP effectively acts as a pixel of the array. The quantum-well layers of a QWIP FPA are usually oriented parallel to the focal plane and therefore perpendicular to the direction of incidence of IR. Based on applicable quantum mechanic selection rules, light polarized parallel to the focal plane cannot photo-excite electrons from the ground state to the excited state of a quantum well. Such light will therefore not be detected by the QWIP.
Thus, with conventional QWIP configurations, absorption quantum efficiency achieved is relatively limited. Net quantum efficiency can be determined by multiplying the absorption quantum efficiency by the photoconductive gain, where the photoconductive gain of a QWIP depends on various design choices made. In addition, light traveling substantially parallel to the focal plane can escape sideways from the QWIP of a given pixel. This escaped light is effectively left out of the detection process, in that its pass through the photosensitive volume of the QWIP is limited. This loss of IR further contributes to low quantum efficiency.
What is needed, therefore, is a QWIP design that provides greater quantum efficiency.